Test device for determining the friction and prestress values of screwed connections

ABSTRACT

The test device for determining the friction and prestress values of screwed connections consists of a transportable device having a central support plate ( 1 ), whereby shearing force sensors and axial force sensors ( 7, 8, 11 ) are arranged on both sides of said plate. The sensors are screwed between an application plate ( 2 ) and a torque receptor plate ( 3 ) and can be replaced or provided with adapters ( 4, 5 ). The sensors ( 7, 8, 11 ) can be extension sensors or piezoelectric sensors. The amplifiers of the piezoelectric sensors can be miniaturized in such a way that they can also be placed in the central support plate ( 1 ). During tightening, the following values are determined: total torque  M tot, head friction torque  M K, thread friction torque  M G, prestress force  F z, to obtain the following values: thread friction value μ G  and head friction value μ K .

[0001] In the automation of screwing operations, especially in automobile and aircraft engineering, the highest quality standards have to be maintained and documented. For this the screwing tools employed are provided with inbuilt torque registering devices guaranteeing exact maintenance of the preadjusted maximum torques or angles of rotation.

[0002] For quality assurance the screwing tools powered pneumatically or electrically are tested at regular intervals on special test benches, after which each tool is given a test certificate.

[0003] With the testing equipment in general use the maximum cutout torques of the screwing tools are tested mainly, and readjusted if necessary. These testing devices function mostly on the basis of strain gauges or piezoelectrics, and are kept in special works proof test stations. They are based on values arrived at in the test laboratories through complicated measurements and calculations to define the relationship between the maximum tightening torque M_(tot) and the screw preload force F_(z). To ensure an optimal non-positive connection these values must be determined for every screw connection.

[0004] The main purpose of the invention is to determine speedily and exactly the principal measured data of a new screw connection using a device according to the invention. For this it must be possible to measure separately the friction values of the screw head F_(RK) on its contact surface and of the screw or nut in the thread F_(RG). This enables material pair values and lubricants to be optimized, so that repeatable screw preload forces F_(z) are obtained, while the unscrewing torques of screw locking devices can be determined and optimized. Important also is the ability to determine quickly and reliably the three measured values

[0005] preload F_(z)

[0006] head friction F_(RK)

[0007] thread friction F_(RG)

[0008] with different material pairs and surface states at different temperatures.

[0009] The invention makes possible a testing device consisting of a central measuring carrier part and interchangeable thread adapters and contact plates which can be screwed together in simple fashion.

[0010] The invention will be described further with reference to six figures:

[0011]FIGS. 1A and B show the two different types of screw connections.

[0012]FIG. 2A usual force/torque diagram based on the angle of rotation α.

[0013]FIG. 3 Functional diagram of the testing device according to the invention.

[0014]FIG. 4 Possible embodiment of the testing device according to the invention shown drawn apart.

[0015]FIG. 5 Typical screw connection to be measured.

[0016]FIG. 6 Measured values determinable with the testing device according to the invention.

[0017] The individual figures may be described as follows.

[0018]FIG. 1A Screw connection on which only the total friction force F_(Rtot) has been measured, according to the state of the art.

[0019]FIG. 1B Another screw connection on which likewise only the total friction force F_(Rtot) has been measured according to the state of the art.

[0020]FIG. 2 Graph showing how the two values preload force F_(z) and total friction force F_(Rtot) are measured with the present testing equipment according to the state of the art.

[0021]FIG. 3 Testing device according to the invention shown schematically, enabling for the first time exact separation of the total friction force F_(Rtot) into

[0022] F_(RK) friction force of screw/nut head and

[0023] F_(RG) friction force of thread.

[0024] This allows for the first time entirely new insights into the analysis of screw connections.

[0025] Mounted on both sides of a carrier plate 1 are sensors 7, 8 and 11 so that they are constrained by means of screws 9 between torque introduction plate 2 and torque sustaining plate 3. Fitted on each side of the carrier plate 1 are both shear sensors 7 and axial force sensors 8, so that a generally symmetrical force distribution results. The shear sensors are arranged so that the individual shear components can be combined for measuring the torque.

[0026] When the test screw 6 is now inserted through the adapter 4 and screwed into adapter 5, upon tightening it the preload force F_(z) is measured by the axial force sensors, the head friction torque M_(K) by the shear sensors 7, and the thread friction torque by the shear sensors 11. If desired, a rotation angle α sensor may be fitted in addition, though this is mostly unnecessary on account of the uncertainty of the beginning of the rotation angle measurement. The four standard values

[0027] preloadforce F_(z)

[0028] total torque M_(tot)

[0029] head friction torque M_(K)

[0030] thread friction torque M_(G)

[0031] which are determined exactly with a single tightening operation in the testing device according to the invention are sufficient for an optimal screw connection design.

[0032] The sensors employed in the testing device may be based on piezoelectrics or strain gauges. Piezoelectric sensors have the advantage that they can be used in a much wider temperature range than strain gauge sensors.

[0033]FIG. 4 shows another embodiment drawn apart. The sensor carrier plate 15 is mounted on the base plate 18. The sensors for torque and axial force are all arranged on the two sides of the carrier plate 15, in which amplifiers may be fitted also, which is advantageous especially with piezoeletric sensors. They enable the measuring signals to be transmitted to the evaluation electronics with low resistance, though this variant narrows the temperature range considerably. The plates 16 and 17 may be changed in simple manner and matched to the particular requirements. They are joined to the sensor carrier plate 15 by the screw connections 21 and 22. The test screw 23 in turn forms the measuring unit together with the plates 16 and 17.

[0034]FIG. 5 shows a test screw 30 once again, with the dimensions from which the principal friction values are derived:

[0035] head friction value $\mu_{K} = \frac{M_{K}}{F_{Z} \times {Dm}}$

[0036] thread friction value $\mu_{G} = \frac{M_{G}}{F_{Z} \times d\quad m}$

[0037]FIG. 6 shows the graph plotted with the four measured values, whereby

[0038] total torque M_(tot)=M_(K)+M_(G)

[0039] leading directly to the peload force F_(z).

[0040] The testing device according to the invention presents in a simple construction new ways for the optimal design of screw connections.

REFERENCE LIST

[0041]FIGS. 1A,B

[0042] F_(z) Preload force

[0043] M_(tot) Total torque=tightening torque

[0044] F_(Rtot)

[0045] Total friction force (thread+contact surface)

[0046]FIG. 2

[0047] α Rotation angle

[0048]FIG. 3

[0049]1 Sandwiched sensor carrier plate

[0050]2 Torque introduction plate

[0051]3 Torque sustaining plate

[0052]4 Adapter insert

[0053]5 Adapter insert

[0054]6 Test screw

[0055]7 Shear force sensors M_(K)

[0056]8 Axial force sensors F_(z)

[0057]9 Sensor screw connections

[0058]10 Base plate

[0059]11 Shear force sensors M_(G)

[0060]FIG. 4

[0061]15 Sandwiched sensor carrier plate

[0062]16 Torque introduction plate

[0063]17 Torque sustaining plate

[0064]18 Base plate

[0065]19 Sensor arrangements

[0066]20 Sensor arrangements

[0067]21 Sensor screw connections

[0068]22 Sensor screw connections.

[0069]23 Test screw

[0070]24 Signal conections F_(z) M_(K) M_(G)

[0071]FIG. 5

[0072]30 Test screw

[0073] Dm Mean diameter of screw head contact surface

[0074] dm Mean thread diameter

[0075]FIG. 6

[0076] M_(tot) Total torque

[0077] M_(K) Head friction torque

[0078] M_(G) Thread friction torque

[0079] μ_(K) Head friction value

[0080] μ_(G) Thread friction value

[0081] F_(z) Preload force 

1. Testing device for screw connections, consisting of screw holding means fitted with sensors, characterized by a sandwiched sensor carrier plate (1, 15) with sensor arrangements on both sides (7, 8, 11, 19, 20) being mounted on one base plate (10, 18) and constrained between the sensors (7, 8, 11, 19, 20) by torque introduction plates (2, 16) and torque sustaining plates (3, 17), detecting separately both the head and thread friction forces and the preload force.
 2. Testing device for screw connections according to claim 1, characterized by the plates (2, 3, 16, 17) being joined with adapters (4, 5) allowing different screw dimensions to be measured with the same device for tightening and loosening torques and also forces.
 3. Testing device for screw connections according to claim 1 or 2, characterized by a number of sensors being arranged in a triangle, square or circle for symmetrical force distribution, disposed in pairs on the two sides of the carrier plate (1, 15).
 4. Testing device for screw connections according to one of claims 1, 2 or 3, characterized by the use of piezoelectric sensors for axial force F_(z) and torques M_(K) and M_(G), whereby sensors measuring pressure and shear simultaneously are suitable.
 5. Testing device for screw connections according to claim 4, characterized by the crystal elements to the piezo sensors being integrated directly in the sandwiched carrier plate (1, 15).
 6. Testing device for screw connections according to one of claims 1 to 5, characterized by electronic amplifiers being integrated directly in the sandwiched carrier plate (1, 15). FIG. 7 Testing device for screw connections according to one of claims 1 to 6, characterized by the electronic ampifiers having range switchover enabling especially interesting parts of the measurements to be magnified by factors. FIG. 8 Testing device for screw connections according to one of claims 1 to 3 and 6 to 7, characterized by the use of measuring elements other than piezoeletric sensors. 